The government is braced for a series of parliamentary defeats this week over controversial plans to reform legal aid.

The House of Lords is expected to vote against key aspects of the legal aid and sentencing bill that critics say will hit some of the most vulnerable in society. Opposition is said to be similar in scale to that which has met the welfare and NHS bills, both of which have already suffered high-profile defeats in the Lords and exposed divisions between the Tories and the Lib Dems.

The Ministry of Justice says legal aid reforms will save £350m and end an ambulance-chasing culture that taxpayers cannot afford. But the Law Society, the independent body that represents solicitors, claims the measures will limit the ability of many people to secure justice, by making the risks of pursuing legal action too high.

Its sister organisation, the campaign group Sound Off for Justice, warns that "at least 650,000 people" will have "nowhere to turn for legal advice" as a result of the reforms, which have also been attacked by a diverse range of groups including Shelter, Netmums and the Women's Institute.

Sound Off for Justice estimates 210,000 families and women who previously would have qualified for legal aid in divorce cases will lose out, while almost 50,000 families and couples who benefit from mediation will also no longer benefit. In addition, 6,000 children under the age of 18, 69,000 vulnerable young adults aged 18 to 24, and 32,250 elderly people would no longer be able to access legal aid. An estimated 135,000 welfare claimants who use legal aid to gain access to legitimate benefits will also miss out.

The government has offered concessions on clauses that would have restricted legal aid for domestic violence and clinical negligence victims. But it is facing more than 77 amendments from peers. Sound Off for Justice claims that, if the bill proceeds in its current form, the government will cut 8% from the criminal legal aid budget, compared with a 29% cut in the family law cases budget and a 53% reduction in welfare cases funding.

A GfK NOP poll conducted for Legal Action Group (LAG), which opposes the reforms, to be published, found 82% of people believed that free advice should be available to those with incomes on or below the national average wage. LAG claims there is rising support in all social classes for employment law advice to be paid for by the state, which it attributes to the public's anxiety over their employment rights due to the economic slowdown.

Steve Hynes, director of LAG, said: "At what point have the government consulted the public on what they want from civil legal aid? The message is very clear from this poll. People believe it is fair for the state to pay for advice on the common legal problems which life can throw at them."

In the introduction to a new report, to be published tomorrow, Gillian Guy, chief executive of Citizens Advice, warns that abolishing legal aid for issues such as welfare benefits, debt, most housing problems and employment will have "a devastating impact" on its ability to provide specialist advice and casework to tens of thousands of people who need help with everyday legal problems.

The Ministry of Justice said: "Victims will be able to receive legal aid to fund the most serious and complex cases, where a 'no win, no fee' agreement is not available, and where the failure to provide funding would breach their human rights."